29 



NOTES ON THE SHELLS OF TRIDACNA, AND DESCEIPTION OF 

 A NEW SPECIES. 



By G. B. SowEKBT, F.L.S. 

 Read Stli December, 1911. 



In my endeavours to sort out a large number of shells of this genus, 

 and to place togetlier the supposed species, 1 have found, as is usually 

 the case in other gtnera, that some of the characters regarded as 

 specific are inconstant, and that intermediate forms link together 

 some of those apparently distinct. I have examined the hinges of 

 a large number of specimens in the expectation of finding some 

 distinctive characters, but in this respect there appear to be no 

 differences excepting such as may be accounted for by age or 

 circumstances of development. The more or less elongated forms of 

 the shells and the number of radiating ribs do not appear to be 

 reliable characters. 



Having brought my investigations to this point, it may seem almost 

 an inconsistency to propose a new species; but the form I propose to 

 call Tridacna acuticostata certainly does seem distinct I'rom all the 

 others. 



The following are the principal works treating upon the genus : — 



{a) Monographs of the Shells. 

 184-5. J. C. Chenu, Illustrations Conchyliologiques, pj). 1-2, pis. i-viii. 

 186'2. L. Eeeve, Conchologia Iconica, vol. xiv, pis. i-viii. 

 1868. H. C. Kiister, Concbylien-Cabinet, pp. 1-7, pis. i-ii. 

 1884. G.B. Sowerby, Thesaurus Conchyliorum, vol. v, pp. 179-82, pis. 48.5-9*. 

 190.3. J. G. Hidalgo, Mem. Real. Acad. Ciencias Madrid, vol. xxi, pp. 382-99. 



(6) Anatomy. 

 186-5. L. Vaillant, Ann. Sci. Nat. Paris, vol. iv, pp. 64-172, pis. viii-xii. 

 1890. A. Menegaux, Eecherches circulation Lamellibranches, pp. 130-4. 

 1899. K. Grobben, Denkschrift. Akad. Wissensch., vol. Ixv, pp. 438-44, pi. 

 1902. H. de Lacaze-Duthiers, Archiv. Zool. Exper., vol. x, pp. 99-212, pi. 

 1904. R. Anthony, Coniptes Eendus Acad. Sci. Paris, vol. cxxxviii, ]3p. 296-8, 

 figs. 



List of Species. 



1. T. GiGAS, Linn. (Sowerby, Thes. Conch., vol. v, p. 179, pi. 188, 

 fig. 11). Of this, largest of allbivalved molluscs, there is a specimen in 

 the British Museum 3 feet in length and weighing 3101b., and specimens 

 have been recorded attaining to even larger dimensions and weighing 

 500 lb." I have not been able with any certainty to trace the young 

 of this species ; the shells supposed to represent it in Reeve's Conch. 

 Icon., figs. \h, c, I regard as very unlikely ; at all events they are 

 inseparable from the diversiform T. elongata. The shell figured and 

 described by Reeve as 1\ g'igas (pi. i, fig. 1) figures in the Thes. 

 Conch, as T. mutica, Lamk., but it is identical with the 2\ gigas as 

 figured in Chenu's Illustrations Conchyliologiques from the Delessert 



^ See Smith, Proc. Malac. Soc, vol. iii, p. 111. 



